As biological photometric devices for measuring a concentration of metabolites in an organ by using light or changes in the concentration, Japanese Patent Application Laid-Open No. 9-135825 discloses a biological photometric device of high sensitivity in a deep portion in an organ and Japanese Patent Application Laid-Open No. 9-98972 discloses a method of forming an image showing functions of human metabolism by using a measurement result. Prior arts will be described hereinbelow on the basis of the methods.
First, taking a change in blood volume in association with brain functions in the cerebral cortex as an example, a method of non-invasively measuring a change in blood volume in an organ will be described with reference to FIG. 12. A light propagation path 1101 shown in the diagram is a propagation path of light emitted from a light source 1102 and reached a light detector 1103 disposed on a scalp 1107 of a subject 1104. In this case, the light detector includes a light source typified by a laser, a light emitting diode, or a lamp and, in some cases, an optical waveguide for guiding light from the light source to the subject 1104.
As shown in FIG. 12, each of the light source 1102 and the light detector 1103 is fixed to a holder 1105 by using a screw 1106. The tip of each of the light source 1102 and the light detector 1103 is in contact with the scalp. The brain is constructed by, in accordance with the order from the surface of the head with which an optical fiber is in contact, the scalp 1107, a skull 1108, a cerebrospinal layer 1109, a cerebral cortex 1110, and the like.
In the biological photometric device, in the case of measuring, for example, brain functions of an adult, the light sources 1102 and the light detectors 1103 are disposed at intervals of 30 mm. The disposition intervals are not limited to 30 mm but are determined in accordance with the structure of brain and optical properties (such as an absorption coefficient and a scattering coefficient) of cerebral substances. The cerebral cortex 1110 is a tissue existing on the inside of the skull and it is known that the cerebral cortex 1110 exists in a region at a depth of about 10 to 15 mm from the scalp in the case of an adult. It is known that the blood volume in the cerebral cortex changes in association with activities of the brain.
As shown by the light propagation path 1101 having the shape of a banana shown in FIG. 12, in the case of setting the disposition interval between the light source and the light detector to 30 mm and detecting a change in blood volume in the cerebral cortex, the sensitivity is the maximum in an about midpoint position between the light source and light detector in the diagram. In the position, according to a simulation of transmission of light in an organ, a light beam becomes the maximum at a midpoint between the light source and light detector. This point is therefore designated as a blood volume change estimating and measuring position. Based on a change in the light amount detected by the light source before and after the blood volume changes, a change in blood volume in the blood volume change estimating and measuring position can be estimated. An example of a method of evaluating a blood volume change is described in detail in Japanese Patent Application Laid-Open No. 9-98972.
A method of forming an image showing the functions of an organ from the result of measurement of the blood volume change will now be described by referring to FIG. 13. Shown in FIG. 13 are: a subject 1201; a measurement area 1202 on the subject; light sources 1203, 1204, 1205, 1206, 1207, 1208, 1209, and 1210 such as semiconductor lasers, light emitting diodes, or lamps which are disposed on light irradiation positions (S1, S2, S3, S4, S5, S6, S7, and S8) on the subject (1201); and photoelectric conversion devices 1211, 1212, 1213, 1214, 1215, 1216, and 1218 typified by avalanche photo diodes and photomultiplier tubes. Light that reached light detection positions on the subject indicated by D1 to D8 in the diagram is led to the photoelectric conversion devices via optical fibers for detection. The light irradiation position and the light detection position are determined at an interval of 30 mm in the case of measuring a metabolite in vivo in the cerebral cortex of an adult.
In the diagram, for example, light reaching the light detection position (D5) in the diagram is light emitted from light irradiation positions (S3, S5, S7, and S6). As described by referring to FIG. 12, the blood volume change estimating and measuring position is in an about midpoint of the light irradiation position and the light detection position. In the measuring method shown in FIG. 13, the number of estimation and measurement positions is 24 (blank circles in the diagram). To form an image showing the concentration of metabolites in an organ or a change in the concentration, first, changes in blood volume are obtained in the positions (24 positions). The changes in blood volume are subjected to two-dimensional spline interpolation, thereby estimating a change in blood volume between the measurement positions and an image is formed by using the result.
According to Neil C. Bruce, “Experimental study of the effect of absorbing and transmitting inclusions in highly scattering media”, Applied Optics, No. 28, Vol. 33, October, 1994 issued by Optical Society of America, it is understood that in the case where the disposition interval between a light source and a light detector is narrow, a larger amount of information of the surface of a light scatterer can be obtained.
In the above-described biological photometric methods disclosed in Japanese Patent Application Laid-Open Nos. 9-135825 and 9-98972, to detect changes in metabolism of an organ tissue in a deep portion of the organ typified by the cerebral cortex, a plurality of light sources and a plurality of light detectors are disposed on a subject at predetermined intervals, a concentration of metabolites in the organ at a plurality of sampling points existing in midpoints between the light sources and light detectors or changes in the concentration is measured, and an image showing the concentration of metabolites in the organ or the changes in the concentration is formed by using the measurement result. In the prior arts, however, an image of only the concentration of metabolites in an organ in a certain depth or the changes in the concentration is formed.